My own take on Boulder Repellent Craters
I’ve been enjoying the boulder repellent thread and the project very much, and after reading Jules’ excellent IOTW last week, I’ve decided to do something different with this week’s. Simply continuing the theme with some of my own ideas and questions.
One thing that seems to be promoting a lot of discussion is the nature of the central feature of these craters that appear to be flat pools of impact melt.
Looking at the latest images in the thread, I personally believe that these central melt pools are not entirely flat/level. To this end I went through the whole thread; I looked at every image, I read every post and many of the links.
Both Anthony Cook and Tom128 have suggested that the melts may not be flat; these were the suggestions I was looking for to follow up this idea.
I’ll briefly explain that as a teen at college I was asked to create seemingly flat surfaces using a thermo-plastic material. One could stand on the surface and assume it was flat. These surfaces were then simply tested with a marble to reveal the direction of the fall or gradient. In the same way, roads and many sidewalks/pavements or sealed exterior ‘foot traffic’ surfaces are not actually level. The gradients are slight (as in, we’re not all struggling to walk in a straight line, falling towards the road), the gradients are revealed in heavy rain where the water can be clearly seen running towards the drains. Of course, this isn’t secret knowledge; I write only to make a point.
The gradient, or fall, does not have to have a high value and can easily be imperceptible to a high flying camera looking straight down but it will have noticeable effects to any rolling rocks and boulders.
Do they all look the same?
If all the features looked the same, I would agree there they may be flat; but there are many obvious differences. Some of the images in the thread clearly show signs of a slight ‘bulge’ in the melt pool that is revealed by the light and dark shades on both sides. So I think it’s fair to assume that at least some are raised slightly towards the centre.
Images posted by Geoff
Another thing that attracts me to the idea of a raised centre is the circular pattern of the boundary around the melt pool in many images. In my opinion, these boulders are simply ‘jammed’ into the ‘low point’. Indeed, many more rocks and boulders can be seen ‘backing up’ the craters walls, being blocked by the embedded ones.
Tom128 points out the circular boundary line
Some rocks and boulders ‘backing up’
Posted by Aliko
I think, also, that the amount of boulders and activity in the area can play another important role in the appearance of these features; assuming the ‘low point’ is a roughly circular area defined in many of these images. If there are enough boulders in the region then the low point can be filled allowing more to simply roll over, closer to the centre. This would explain the jagged shaped boundary lines around the centres with an apparently smaller central melt pool. In some cases, it appears that a few have bounced over the boundary to land quite close to the centre.
posted by Half65
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Posted by Cruuux
I’m certainly not an expert in the mechanics of melt pools and my knowlege of lunar volcanism is limited. If these features were volcanic then I suppose it would be fair to expect some bulging towards the centre. However, as these pools are are formed from impact melt, different processes are at work.
Now, all of the above is written in my humble opinion to throw out an idea, get some feedback, promote discussion and/or take a beating.
Thanks for taking the time to read this.
Thomas 
Thomas J is a volunteer moderator for the Moon Zoo forum.
Moon Zoo Science Conference Abstract Now Available
Full Moon - Credit, Stuart Robbins
Hello Zooites! This is a quick post to let you know that I’m headed to the Planetary Crater Consortium conference in Flagstaff, AZ, in about three weeks and I’ll be presenting a paper there about Moon Zoo and what we’ve learned so far. The abstract is two pages and it’s not too technical. If you’re interested in reading it, you can download the ~140 KB PDF here!
Boulder Repellent Craters
Something is puzzling us on the Moon Zoo forum. Ever since Tony Cook set us a challenge last year to find craters with floors cleared of boulders we have been collecting these boulder repellent craters with melt pool floors. Tony Cook said of one of the examples:
“Why have the numerous boulders within this crater avoided filling the centre of this crater? Why is the central area so featureless – presumably it is younger than the main crater? Or is it that the solar altitude of 56° is preventing us from seeing craterlets on the floor of the flat patch. Is this central patch the reverse of a central peak, perhaps a central dimple and was filled with impact melt?”
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So why is this a feature of just some craters and not others? Discussions on the forum have raised several questions. Some of these melt pools appear to show signs of impacts before the melt had solidified. So did the melt pool solidify at a slower rate so that some of the boulders that did roll into the centre sank from view? Although lunar temperatures suggest that melt pools would have solidified quickly. Are these cleared areas just very flat so the boulders stop rolling when they meet it? Does this feature correlate with a particular size of crater, impactor or type of rock being hit? And what role does space weathering play?
Some guidance was provided by a forum member xitehtnis whose work includes boulder clustering on Mars. He advised us that there are similar craters on Mars and offered some current thinking stating that different reasons applied depending on the age and environment of the craters:
“ For fresh craters, some amount of melt is generated in the course of an impact that takes diffusivity dependent cooling timescales for different depths of melt (which scales to crater size) (see Melosh, 1989).
For highly degraded craters it is likely the regolith has all been broken up to pieces beyond the limit of resolution due to impact gardening (very small impactors break up boulders and generally resurface the moon at small scales) ……..
For craters in the middle there could be a wide array of things going on. My preferred hypothesis comes from previous studies I worked on regarding glacial moraines. Basically, the idea is that fine particles are more easily mobilized during any erosive process. Since most erosive processes are gravity driven and craters generate slopes you would expect small particles to migrate to the lows in slopes while leaving the large particles (boulders) behind (check out Putkonen, Connolly, and Orloff 200?).”
xitehtnis also pointed us to this paper Impact Melt In Small Lunar Highlands Craters (Plescia et al, 2011) which notes that such melt deposits are very rare in small (km to sub-km) simple craters and concludes that:
“It may be that the small craters for which well defined melt pools are observed represent a special case – a vertical or near vertical impact.”
Could this also be the case for larger craters? OK we have more questions than answers right now! But that’s good. It allows us to research and learn – the forum is a great place for that. So we will continue to look for these intriguing craters, maybe map them out – and continue to debate their formation.
With thanks to the following forum members for their contributions: Tom128, JFincannon, astrostu, xitehtnis, Geoff, IreneAnt, Caro, matt.vader, jules, Half65, ElisabethB, Cruuux, Aliko, Thomas J, claymore, khearn. Read more and contribute here.
jules is a volunteer moderator for the Moon Zoo forum
Ejecta Blocking Boulder
On the floor of Al-Bakri crater lies an approximately 100 foot diameter crater with a very unusual feature that may have interfered with its impact ejecta pattern.
You can see the open wedge area (no ejecta) caused from a strategically placed boulder near its rim. Here is what Moon Zoo Team member Irene Antonenko ( IreneAnt ) has to say:
” This is really interesting because the boulder really shouldn’t affect the placement of the ejecta all that much. When the crater is being formed, ejecta is being thrown out from closer to the centre, so the boulder would be too far away to affect the ejection of material. And, when the material lands, it is thought to land from above (not sideways), so again, the boulder shouldn’t affect the emplacement of ejecta material. “
There may be some tells as one looks around the crater floor for clues that may help solve this mystery. Our ejecta blocking boulder appears to have been sitting at that location for quite some time. There does not appear to be any boulder tracks like the one below from a boulder that probably rolled down from the rim of Al-Bakri crater south of our mystery boulder.
If you look at the other boulders in the area you will see that they appear to be white (high albedo). The ejecta blocking boulder is darker and appears to be the black sheep of the boulder community. This anomaly may be resolved if one considers that the darker color of the boulder may be due to it being covered with a dusting of fine ejecta particles from the impact.
It appears that the major force of the impact, as Irene stated above, moved up and over the boulder not affecting or having little affect on its placement. However, there does seem to be a lateral surface flow of ejecta that hit the boulder. The boulder then acted like a nozzle/guide that created the unique ejecta- free wedge design out from the crater. There is also a possibility that crater excavation moved our mystery boulder up to its present position while blocking the ejecta along the way.
The old boulder may have staked its claim to that spot first and was not willing to give it up even with the more recent arrival of a claim jumping impactor. What are your thoughts?
Tom128 is a regular contributor on the Moon Zoo forum.
The Compton-Belkovich Thorium Anomaly
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A few days ago Moon Zoo forum moderator Geoff posted some links to articles on lunar volcanism on the far side. Evidence of basaltic far side volcanism is already fairly well documented but this was different in that it was further evidence of silicic volcanism in a particular area of interest on the lunar far side.
Basaltic lava eruptions are generally effusive and fluid. Silicic eruptions are associated with extremely viscous lava and explosive volcanism. Non-basaltic lunar volcanism is quite rare. Examples of near side silicic volcanism have been identified in Mare Procellarum.
The Compton-Belkovich region on the northern far side (Longitude: 99.8° Latitude: 61.6°) lies between two very large ancient impact craters, Compton and Belkovich, and was found to contain large concentrations of the radioactive chemical element thorium back in 1998 when it was detected by the gamma-ray spectrometer instrument on the Lunar Prospector spacecraft. Known since as the Compton-Belkovich Thorium Anomaly (CBTA) it has remained a bit of a mystery which the Lunar Reconnaissance Orbiter is helping to unravel.
The topography of the area includes a series of domes ranging from around 1 km to 6 km in diameter, some depressions which could be evidence of collapsed volcanic features and it is also a highly reflective region. The steeply sloping sides of some of the domes are thought to have been the product of slowly flowing viscous lava. And the fact that there are few impact craters in this area indicates that, in lunar terms, this is a fairly recent fresh feature.
3D Digital Terrain Model
Follow the reference links below and the forum thread to read more about this fascinating region of the Moon.
references:
NAC image M119198897RC
42nd Lunar and Planetary Science Conference (2011)
Jules is a volunteer moderator for the Moon Zoo forum
Moon Zoo 
